Question

这里涉及2个课程................ 来自另一个类（SQLitedatabase）的方法：

public String QuantReturn(){

        Testing x = new Testing();
        String code = x.QRcodeReturn();

        String quantity=null;
        String selectQuery = "SELECT quantity FROM meds_db WHERE name = " + code;
        Cursor c = db.rawQuery(selectQuery, null);

        if(null != c && c.moveToFirst()){
            quantity  = c.getString(c.getColumnIndex(KEY_QUANTITY));
        }
        return quantity;
    }

和被叫时崩溃的类........................................ ..........

public class Testing extends Activity {


public static final String SCAN_RESULT = "MyPreferencesFile";

public String QRcodeReturn(){

    SharedPreferences codeHack = getSharedPreferences(SCAN_RESULT,0);
    String QRcode = codeHack.getString("entry", "unregistered");

    return QRcode;
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_testing);


         ToolDB db = new ToolDB(this);
         String c1 = db.QuantReturn();

         TextView t3 = (TextView) findViewById(R.id.slot4);
         t3.setText(c1);

}

因崩溃而发生的错误的Logcat：

03-19 13:59:08.517: E/AndroidRuntime(10579): FATAL EXCEPTION: main
03-19 13:59:08.517: E/AndroidRuntime(10579): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.SurgicalMate/com.example.SurgicalMate.Testing}: java.lang.NullPointerException
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2351)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.ActivityThread.access$600(ActivityThread.java:151)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1331)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.os.Looper.loop(Looper.java:155)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.ActivityThread.main(ActivityThread.java:5454)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at java.lang.reflect.Method.invokeNative(Native Method)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at java.lang.reflect.Method.invoke(Method.java:511)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1029)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:796)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at dalvik.system.NativeStart.main(Native Method)
03-19 13:59:08.517: E/AndroidRuntime(10579): Caused by: java.lang.NullPointerException
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:153)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at com.example.SurgicalMate.Testing.QRcodeReturn(Testing.java:23)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at com.example.SurgicalMate.ToolDB.QuantReturn(ToolDB.java:76)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at com.example.SurgicalMate.Testing.onCreate(Testing.java:38)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.Activity.performCreate(Activity.java:5066)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1101)
03-19 13:59:08.517: E/AndroidRuntime(10579):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
03-19 13:59:08.517: E/AndroidRuntime(10579):    ... 11 more

提前谢谢

Answer 1

您必须在调用onCreate后访问共享首选项。否则上下文将为null。

Answer 2

尝试这种方式：

public String QRcodeReturn(){

    SharedPreferences codeHack = getSharedPreferences(SCAN_RESULT,0);
    if (codeHack != null) {
        String QRcode = codeHack.getString("entry", "unregistered");
        return QRcode;
    } else {
        return "";
    }
}

因为我看到的是， codeHack 为空，您尝试调用null对象的 getString 。

Answer 3

你的getSharedPreferences（）方法返回null - 因此，codeHack解析为null，对此String的进一步操作抛出空指针异常。最有可能的是，您的首选项文件尚未正确设置。

Android：为什么我的应用程序在调用此类时会崩溃？

3 个答案: